Ultrasound imaging is used in a wide variety of situations and has particular application in medical imaging. Great strides have been made in the quality of images provided by ultrasound imaging devices over the last 20 years however there remains room for improvement, particularly in the areas of image quality, the quality of information provided in ultrasound images and imaging frame rate.
In conventional ultrasound, a transducer with multiple elements is used to transmit and receive ultrasonic energy. Typically each element of the transducer can be controlled individually. On transmit, the timing and amplitude of each transmit pulse from each element can be controlled individually. The timing is chosen such that signals from the elements arrive at a predetermined location within the target at the same time. This process of adjusting the delays is called transmit focusing. On receive, the signals from each element are typically digitized, multiplied by a factor and summed with other elements to achieve continuous receive focusing along a line of interest.
The amplitudes of transmit pulses and/or receive pulses may be adjusted. The process of adjusting the amplitude of transmit and/or receive pulses is called apodization.
In conventional ultrasound the transmit focus is scanned to different locations in a medium being studied. The medium may, for example, comprise tissues of a human or animal. In general, the medium of interest is on one side of the transducer. This side may be called the ‘front’ side of the transducer. The transmit foci are placed in front of the transducer. The resolution of such conventional ultrasound images is best in the vicinity of a transmit focal point. Hence, as the receive beam is continually focused, the best resolution will occur when the receive focus is located around the transmit focal point. To improve image quality, multiple transmit focal points may be placed along a specific direction. While this can help to improve image quality, multiple transmissions come at the cost of lower frame rate as each focal point requires a separate transmission.
Synthetic aperture ultrasound imaging techniques have been investigated as an alternative to conventional techniques. In an example synthetic aperture technique, a single element is used for transmission. On receive, the data from multiple elements are digitized and stored. The process is repeated for all elements in the transducer array. The frame rate achieved in this technique is quite low and the technique is prone to motion artifacts. However, for a stationary target, an image can be formed which is continually focused in transmit and receive. Another significant disadvantage of single element synthetic aperture is that that the sensitivity is quite low—hence the depth of penetration is also typically quite low.
Some references describe the use of plane waves, unfocussed waves, weakly focused waves or divergent waves in the field of ultrasonic imaging. For example, U.S. Pat. No. 6,551,246 describes the use of plane waves with some limited discussion of unfocussed or weakly diverging and weakly focused waves. US2009/0234230, describes the use of plane waves with the addition of a “coherence enhancing step”. “Synthetic Aperture tissue and flow ultrasound imaging” by Svetoslav Ivanov Nikolov [1], describes creation of diverging waves. “Synthetic Aperture Imaging for Small Scale Systems” by Karaman et. al. [2], describes transmitting with a defocussing delay profile. The transmit delay profile is chosen such that the resulting lateral response approximates the shape of a single element placed at the center of the active aperture.
There remains a need for ultrasound apparatus and methods capable of providing improved images and/or providing a cost-effective alternative to existing high quality ultrasound imaging systems.